feat(mcp): re-export UnauthorizedError so hosts can detect auth failures

[siarheidudko]

Why

A host that wants to react to a mid-session authorization failure — the one case where the transport gives up and a human has to re-authorize — had no way to recognise it.

The SDK's UnauthorizedError extends Error but never assigns this.name:

export class UnauthorizedError extends Error {
    constructor(message) { super(message ?? 'Unauthorized') }
}

so err.name is "Error". A host checking err.name === 'UnauthorizedError' silently never matches, and matching on the message would also catch a tool whose own error text happens to say "unauthorized" (our connector throws the server's text verbatim for isError results). Without the class itself, there is no reliable handle.

Found by review of the React bindings (dudko-dev/agent-web-react#3), where that check was dead code: the panel kept reporting "connected" while every tool call failed.

What

./mcp re-exports UnauthorizedError from the SDK, so an identity check runs against the same module instance the connector uses. One line of surface; no behaviour change.

The accompanying test pins the assumption this export exists for — if the SDK ever starts setting name, the test says so rather than leaving a silently redundant export behind.

typecheck, format:check, build, test (90 passing) are green. Version bumped to 0.0.12 so the release workflow publishes it.

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